It looks like they did a Google image search for whatever the subject of the book is, and slap the first thing they find on the cover.

Example: The Agile Samurai https://imagery.pragprog.com/products/176/jtrap.jpg?12985898...

Dart 1 For Everyone is even better. https://imagery.pragprog.com/products/432/csdart1.jpg?140865...